Formation of films for organic photovoltaics

ABSTRACT

A reaction comprising an organic Zn precursor in the amounts of (1−y), an organic Sn precursor in the amounts of y and a base in the amount of (1−y). The product produced from the reaction is (SnO x ) y ZnO (1-y) .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Non-Provisional application which claims thebenefit of and priority to U.S. Provisional Application Ser. No.62/235,855 filed Oct. 1, 2015, entitled “Formation of Films for OrganicPhotovoltaics,” which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to the formation of films for organicphotovoltaics.

BACKGROUND OF THE INVENTION

The present disclosure generally relates to organic solar cells andsimilar electronic devices. Today's increasing demand for renewableenergy resources, especially solar power, is driving researchers todevelop low cost, efficient photovoltaic devices. Organic photovoltaics(OPVs) are an attractive route toward solving the terawatt energyproblem.

Solution processed organic photovoltaics have the potential to become alow-cost photovoltaic technology. OPVs can be fabricated on flexiblesubstrates in a roll-to-roll process, which may enable photovoltaics toenter entirely new markets. One of the milestones for commercializationof OPVs is improving device efficiencies, which reduces overall cost.One way of improving device efficiency is through utilizing interfacialcharge transport layers.

Interfacial charge transport layers sandwich the photoactive layer anddetermine the device polarity, help to collect charges, and transportthe charges to the electrodes. Materials for these charge transportlayers can be transparent, have low resistance and be chemically stable.The electron transport layer collects and transports electrons mainlygenerated from the acceptor to the cathode. A low work functioninterface is required to make Ohmic contact with the organic photoactivelayer. ZnO is a common n-type semiconductor used as an electrontransport layer in OPVs. ZnO can be processed from a sol-gel solution,and when combined with post-annealing, produces crystalline thin filmsat low temperatures (150-200° C.). Tin oxide (SnO_(x)) is known to behighly transparent and conductive but higher post-annealing at 350-500°C. is often required. In addition, SnO_(x) has one of the lowest oxygenand water vapor transmission rates for metal oxides, which may aid inorganic electronic device stability.

The low work function of ZnO and high conductivity of SnO_(x) have beenpreviously combined to form mixed metal oxides whereby the ratio ofZn:Sn affects the electronic and physical properties of composites.These zinc tin oxides have application in a number of areas includingtransistors, sensors, transparent conductors and organic photovoltaics.

There exists a need for a new low temperature sol-gel solutionprocessing technique for preparing zinc tin oxides with tunablecomposition.

BRIEF SUMMARY OF THE DISCLOSURE

A reaction comprising an organic Zn precursor in the amounts of (1−y),an organic Sn precursor in the amounts of y and a base in the amount of(1−y) to 1. The product produced from the reaction is(SnO_(x))_(y)ZnO_((1-y)).

A reaction comprising an organic Zn precursor comprising Zn(CH₃CO₂)₂ inthe amounts of (1−y), an organic Sn precursor comprising Sn(CH₃CO₂)₂ inthe amounts of y, and a base comprising alkanolamine in the amount of(1−y) to 1. The reaction occurs at a temperature less than 225° C. toproduce (SnO_(x))_(y)ZnO_((1-y)).

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefitsthereof may be acquired by referring to the follow description taken inconjunction with the accompanying drawings in which:

FIG. 1 depicts an inverted device architecture.

FIG. 2 depicts the effect of SnOx content in SnOx:ZnO mixed metal oxideelectron transport layers.

FIG. 3 depicts the transmittance of SnOx, 15% SnOx and ZnO films onglass substrates.

FIG. 4 depicts the effect of annealing temperature on the powerconversion efficiency of OPV devices with the following architecture:ITO/(SnOx)0.15:(ZnO)0.85/P(BDTE-FTTE)/MoOx/Ag.

FIG. 5 depicts the UPS spectra of ITO, SnOx, ZnO and the mixed metaloxide ZTOs.

FIG. 6 depicts the energy diagram illustrating the tunable work functionof SnOx:ZnO mixed metal oxide composites in alignment with the otherlayers in the OPV device stack

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement orarrangements of the present invention, it should be understood that theinventive features and concepts may be manifested in other arrangementsand that the scope of the invention is not limited to the embodimentsdescribed or illustrated. The scope of the invention is intended only tobe limited by the scope of the claims that follow.

The present embodiment discloses a reaction of an organic Zn precursorin the amounts of (1−y), an organic Sn precursor in the amounts of y;and a base in the amount of (1−y) to 1. The resultant product is(SnO_(x))_(y)ZnO_((1-y)).

In one embodiment the organic zinc precursor comprises Zn(CH₃CO₂)₂ orZn(CH₃CO₂)₂*2H₂O.

In one embodiment the organic tin precursor comprises Sn(CH₃CO₂)₂.

In another embodiment the base is an alcohol. Examples of bases that canbe used including amines or alkanolamines.

In yet another embodiment, the reaction also comprises a solvent. Thesolvent can be used to dissolve either the zinc precursor or the tinprecursor. One example of a solvent that can be used is water, alcohol,aminoalcohol, carboxylic acid, glycol, hydroxyester, aminoester or amixture. Some examples include: 2-methoxyethanol, methanol, ethanol,propanol, butanol, pentanol, hexanol, ethyleneglycol, ethoxyethanol,methoxyethanol, ethoxypropanol, ethoxyethanol, dimethyloxyglycol,N,N-dimethylformamide.

In one embodiment, (SnO_(x))_(y)ZnO_((1-y)) is used as an electrontransport layer for an organic photovoltaic device. In anotherembodiment the organic photovoltaic devices has an inverted devicearchitecture. An inverted device architecture has the positive andnegative electrodes reversed. FIG. 1 depicts an inverted devicearchitecture which employs indium tin oxide as the cathode and silver asthe anode. In this type of device, it is theorized that electrons needto move from the polymer:fullerene active layer to the cathode.Electrons are transported from the photoactive layer by the electrontransport layer, and extracted to the transparent cathode.

In one embodiment, (SnO_(x))_(y)ZnO_((1-y)) is a sol-gel solution.

In another embodiment, (SnO_(x))_(y)ZnO_((1-y)) was prepared bydissolving zinc acetate dihydrate or tin(II) acetate in 2-methoxyethanoland ethanolamine. One example of the reaction is shown below:

EXAMPLES Solution Formation

A ZnO sol-gel solution was prepared by mixing 0.33 g Zn(CH₃CO₂)₂ in 3 mLof 2-methoxyethanol with 92 μL of ethanolamine. Similarly SnOx sol-gelsolutions were prepared by dissolving 0.36 g of Sn(CH₃CO₂)₂ in 3.5 mL of2-methoxyethanol, and 99 μL of ethanolamine. ZnO & SnOx were studiedindependently and as a mixed metal oxide system. Mixed sol-gel solutionswere prepared from stock zinc and tin solutions. The amount of Sn in themixed solution could be (5, 10, 15, 70, 95) vol %. In this embodimentthe solutions were stirred for at least an hour before spin casting onindium tin oxide.

Device Fabrication

The photoactive layer consisted of the donor polymerpoly(4,8-bis(5-2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate(P(BDTE-FTTE)) and acceptor [6,6]-phenyl-C70-butyric acid methyl ester(PCBM) at a ratio of 1:1.6, respectively. The total solutionconcentration was 26 mg/mL in o-xylene. The photoactive layer solutionwas stirred and heated at 80° C. overnight in a nitrogen filled glovebox. The next day 2.5 vol % of 1,8-diiodooctane was added and thesolution was heated on the hot plate at 80° C. for an hour. The solutionwas then filtered with a 2.7 μm glass fiber syringe filter.

Indium tin oxide patterned glass substrates were cleaned by successiveultra-sonications in detergent, deionized water, acetone, andisopropanol. Each 15 min step was repeated twice and the freshly cleanedsubstrates were left to dry overnight at 80° C. Preceding fabrication,the substrates were further cleaned for 30 min in a UV-ozone chamber andthe electron transport layer was immediately spin coated on top.

Single component or mixed metal oxide solutions were filtered directlyonto indium tin oxide with a 0.25 μm poly(tetrafluoroethylene) filterand spin cast at 5000 rpm for 40 s. Film were then annealed at 220° C.for 15 min, and directly transferred into a nitrogen filled glove box.ZnO films were annealed at 170° C.

The photoactive layer was deposited on the electron transport layer viaspin coating at 1200 rpm for 40 s and directly transferred into a glasspetri dish to solvent anneal for 1 h. After solvent annealing, thesubstrates were loaded into the vacuum evaporator where MoOx (holetransport layer) and Ag (anode) were sequentially deposited by thermalevaporation. Deposition occurred at a pressure of 1×10-6 torr. MoOx andAg had a thickness between 10 nm and 200 nm, respectively. Samples werethen were then encapsulated with glass using an epoxy binder and treatedwith UV light for 3 min.

Table 1 depicts the photovoltaic parameters of ZnO and SnOx electrontransport layer with the following device architecture:ITO/ETL/P(BDTE-FTTE):PCBM/MoOx/Ag.

TABLE 1 Work Jsc Voc FF PCE Rs Rsh Function ETL (mA/cm²) (V) (%) (%) (Ωcm²) (Ω cm²) (eV) ZnO 15.1 0.774 61.6 7.21 9.88 816 3.75(SnO_(x))_(0.05)(ZnO)_(0.95) 14.8 0.760 55.2 6.67 5.06 288 3.68(SnO_(x))_(0.15)(ZnO)_(0.85) 16.0 0.779 66.9 8.28 5.64 832 3.74(SnO_(x))_(0.75)(ZnO)_(0.25) 15.6 0.713 55.8 6.17 12.4 623 3.93(SnO_(x))_(0.95)(ZnO)_(0.05) 15.8 0.737 61.3 7.08 15.4 476 4.13 SnO_(x)15.7 0.757 62.3 7.41 6.87 769 4.15

Using ZnO as the electron transport layer resulted in an average powerconversion efficiency (PCE) of 7.21%, compared to the average powerconversion efficiency of SnOx of 7.41%. The tin oxide ETLs had highershort-circuit current density (Jsc) and lower series resistance (Rs)which can be attributed to its superior transparency and conductivityproperties, respectively. ZnO devices had higher open-circuit voltages(Voc) presumably due to better interfacial energy alignment with thephotoactive layer as a result of its lower bulk work function.

Mixed Metal Oxide Electron Transport Layers

In order to determine whether there is any effect of combining low workfunction ZnO with conductive SnOx, a range of mixed metal oxidecompositions were prepared, where the SnOx component ranged from 5% to95% (by volume). As the SnOx content increased, there is a peak inphotovoltaic performance at 15% SnOx. On either side of 15% SnOx theperformance drops significantly as seen in FIG. 2.

The high conductivity of tin oxide and the high transparency of ZnO havebeen combined at an optimal ratio of (SnOx)0.15(ZnO)0.85, which resultedin an average PCE of 8.28%. This is significantly higher than thephotovoltaic performance of the pure ZnO or SnOx thin films. Thiscomposition had the best performance in all photovoltaic parametersexcept for the Rs. In general, ZTOs with a higher SnOx content had asuperior Jsc, likely the result of higher transparency, but had a lowerVoc due to the higher work functions of SnOx rich composites, asreported in the literature.

To further optimize the processing conditions for 15% SnOx, the filmswere annealed at 170, 200 and 220° C. to elucidate the effect on thephotovoltaic performance. Table II depicts photovoltaic parameters ofdifferent annealing temperatures on (SnOx)_(0.15):(ZnO)_(0.85) electrontransport layer organic photovoltaic devices.

TABLE II Annealing Temperature Jsc Voc FF PCE Rs Rsh (° C.) (mA/cm²) (V)(%) (%) (Ω cm²) (Ω cm²) 170 15.8 0.801 61.1 7.72 5.39 624 200 16.2 0.80463.6 8.29 5.35 759 220 16.6 0.803 63.4 8.45 5.30 754

At 170° C., the average PCE of devices was 7.72%. By increasing the ETLannealing temperature to 200° C. and 220° C., the PCE increased to 8.29%and 8.45%, respectively. A major contributor to the increase in PVperformance was the increase in the Jsc and FF. At lower annealingtemperatures, the ZTO composite likely has poor crystallinity, whichimproves with higher annealing temperatures. However, by annealing theelectron transport layer at 220° C., device efficiencies up to 8.99%were attained. The sol-gel synthesis for ZTO thin films is able toobtain high performance at significantly lower annealing temperaturescompared to the standard ZnO films. Annealing the electron transportlayer at lower temperatures is beneficial when transferring processingto flexible plastic substrates and roll-to-roll processing.

Optical Properties of Zinc Tin Oxide Films

As light must first pass through the electron transport layer to thephotoactive layer to generate charges, high transparency of the filmacross the solar spectrum is critical. The transparency of SnOx, ZnO,and 15% SnOx was characterized on glass substrates and the transmittancespectra are presented in FIG. 3. It is common for the scientificcommunity to benchmark the transmittance of transparent conductors at550 nm. From FIG. 3, the transparency of the 15% SnOx sample is superiorto the single component metal oxides, reaching 98.8% transparency at 550nm. ZnO and SnOx films are 96.9% and 95.2% transparency at 550 nm. Thesuperior optical properties of 15% SnOx are reflected in this compositeobtaining the highest average Jsc in OPV devices. Allowing more photonsto pass through the cathode and electron transport layer can increasethe absorption of the photoactive layer, resulting in higher Jsc.

The ZnO film shows an excitonic peak at ˜346 nm, which is characteristicof small ZnO crystallites. When adding 15% SnOx to ZnO, the peak shiftsto higher energy at ˜325 nm. This blue-shift is characteristic with areduction in the crystallite size of ZnO by the addition of SnOx. Thepeak width is significantly wider, indicating a higher degree ofpolydispersity of ZnO crystallite sizes in these films. The SnOxspectrum is nearly featureless with a very small electronic transitionat ˜475 nm. As this peak intensity is very small, the SnOx film has alow degree of crystallization. As shown in FIG. 4, higher annealingtemperatures may increase the crystallinity of SnOx and mixed ZTO films,however higher temperatures must be balanced by processing cost andadaptability to flexible plastic substrates.

Work Function of Mixed Metal Oxide Films

To understand the role of SnOx in the mixed metal oxide films wedetermined the work function of the films using ultravioletphotoelectron spectroscopy (UPS). UPS is analogous to X-rayphotoelectron spectroscopy (XPS) but uses ultraviolet radiation instead.Since the power of UV light is lower than X-rays, UPS is even moresurface sensitive compared to XPS. As such, UPS typically characterizesthe top 1-3 nm surface of films. In photoelectron spectroscopy, theaddition of UV energy (hv) and kinetic energy (KE) of emitted electronsis equivalent to the binding energy (BE) of electrons within a specificatomic orbital. This is formalized into the following equation:

BE=KE+hv  (1)

UPS detects both photoelectrons and secondary electrons. The cutoff ofthe secondary electron peak at high binding energy is concomitant withthe film's surface work function, which is the minimum amount of energyrequired to remove an electron from a film to vacuum. The work functionof anodes, cathodes and carrier transport layers is critical in organicphotovoltaics as it determines the device's polarity, as well as carrierextraction efficiency.

The UPS spectra of mixed metal oxide films are seen in FIG. 5. Thespectra are plotted showing the secondary electron cutoff region wherethe work function is determined. The work function of the ITO cathode is4.65 eV. In order for ITO to collect electrons, the work function mustbe lowered to increase electron specificity. Both ZnO and SnOx decreasethe work function to 3.75 eV and 4.15 eV, respectively. For compositefilms at 95% and 70% SnOx, the work function is in between the singlecomponent metal oxide and is 4.13 eV and 3.93 eV. Further decreasing theSnOx content reduces the work function lower than ZnO-only films toreach 3.73 eV at 30% SnOx and 3.68 eV at 5% SnOx. These two compositionsalso have the lowest Rs as seen in Table I, which indicates a reductionof resistive losses in the OPV devices. Further reducing the workfunction beyond that of ZnO is particularly interesting as the mixedmetal combination obtains different physical properties compared to theindividual materials on their own.

An energy band diagram for the organic photovoltaic device architectureis presented in FIG. 6. The figure reiterates that decreasing SnOxcontent in ZTO films reduces the work function. Ideally the workfunction of the electron transport layer should be less than the lowestunoccupied molecular orbital energy of PCBM. This is the case for 15%and 5% SnOx as well as 100% ZnO. For the ZTO composites, lowering thetin content to 15% improves photovoltaic performance as a result ofimproved interfacial energy alignment. ZTO composites less than 15% tinhave a reduction in performance. This may be the result of lowering thefilm conductivity with increasing zinc content.

ZTO films contain 15-20 atomic % of acetate as characterized with X-rayphotoelectron spectroscopy. Table III below depicts the atomicconcentration of ZTO films cast from 65% and 35% diluted sol-gelsolutions.

TABLE III 65% Room 35% Temperature 170° C. 210° C. 240° C. 170° C. 210°C. O 43.6 43.6 44.3 45.3 43.4 44.3 C—C 17.6 14.9 12.7 10.4 20.4 25.2COOH 11.3 10.3 8.5 6.3 4.8 3.4 Zn 26.0 28.9 31.4 35.0 27.7 22.4 Sn 0.71.4 2.1 2.2 3.8 4.8 N 0.8 0.9 1.1 0.8 — —

In closing, it should be noted that the discussion of any reference isnot an admission that it is prior art to the present invention,especially any reference that may have a publication date after thepriority date of this application. At the same time, each and everyclaim below is hereby incorporated into this detailed description orspecification as an additional embodiment of the present invention.

Although the systems and processes described herein have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made without departing from the spirit and scopeof the invention as defined by the following claims. Those skilled inthe art may be able to study the preferred embodiments and identifyother ways to practice the invention that are not exactly as describedherein. It is the intent of the inventors that variations andequivalents of the invention are within the scope of the claims whilethe description, abstract and drawings are not to be used to limit thescope of the invention. The invention is specifically intended to be asbroad as the claims below and their equivalents.

1. A reaction comprising: an organic Zn precursor in the amounts of(1−y); an organic Sn precursor in the amounts of y; and a base in theamount of (1−y) to 1, to produce (SnO_(x))_(y)ZnO_((1-y)).
 2. Thereaction of claim 1, wherein the organic Zn precursor comprisesZn(CH₃CO₂)₂*2H₂O.
 3. The reaction of claim 1, wherein the organic Snprecursor comprises Sn(CH₃CO₂)₂.
 4. The reaction of claim 1, wherein thebase is an alcohol.
 5. The reaction of claim 1, wherein the base isalkanolamine.
 6. The reaction of claim 1, wherein the reaction alsocomprises a solvent.
 7. The reaction of claim 1, wherein the solvent is2-methoxyethanol.
 8. The reaction of claim 1, wherein the reactionoccurs at a temperature above room temperature.
 9. The reaction of claim1, wherein the reaction occurs at a temperature greater than 150° C. 10.The reaction of claim 1, wherein the reaction occurs at a temperatureless than 250° C.
 11. The reaction of claim 1, wherein the reactionoccurs at a temperature less than 225° C.
 12. The reaction of claim 1,wherein (SnO_(x))_(y)ZnO_((1-y)) contains from about 10 to 25% atomic %of acetate as characterized with x-ray photoelectron spectroscopy. 13.The reaction of claim 1, wherein (SnO_(x))_(y)ZnO_((1-y)) is used as anelectron transport layer for an organic photovoltaic device.
 14. Areaction comprising: an organic Zn precursor comprising Zn(CH₃CO₂)₂ inthe amounts of (1−y); an organic Sn precursor comprising Sn(CH₃CO₂)₂ inthe amounts of y; and a base comprising alkanolamine in the amount of(1−y), wherein the reaction occurs at a temperature less than 225° C. toproduce (SnO_(x))_(y)ZnO_((1-y)).